U.S. Pat. No. 4,481,910 discloses a two-stroke combustion engine as described by way of introduction. In this engine, what is referred to as stratified charge is provided by supplying to the combustion chamber more or less clean air and a fuel-air mixture from a carburettor which is connected to the crankcase. The carburettor comprises a valve, which opens at a negative pressure in the crankcase and closes at a positive pressure in the same. When a positive pressure occurs in the crankcase, fuel-air mixture is forced out of the crankcase through a short suction duct and a long air duct. Since, during a preceding engine cycle with a negative pressure in crankcase, the air duct has been filled with clean air through an extra air shutter, almost clean air is forced through the air inlet into the cylinder of the engine and, in this way, the cylinder is scavenged with a minimum of fuel leakage. This results in improved fuel economy as well as cleaner exhaust gases.